Both weapons are considered "500-pound" munitions because they're based on a standard 500-pound bomb known as the Mk 82. The body of an Mk 82 is augmented with a fuze, a guidance system, and other accessories like wings and tail fins; all of these add to its total weight.

But the Mk 82 itself packs only about 200 pounds of explosives. It gets the rest of its weight from a steel case, about half an inch thick, that helps to penetrate targets and provides material for fragmentation.

The "500-pound" designation doesn't give you much information about how powerful a given weapon might be. It's more useful for figuring out how many bombs will fit onto your aircraft. That's because different types of 500-pound bombs have different amounts of explosive inside—one might be designed for bunker-busting and another for fragmentation. (The Mk 82 is an example of "general purpose" ordnance.) You can get a better sense of a weapon's destructive potential by looking at its "explosive yield." That tells you how strong an explosion the bomb will make, in terms of equivalent pounds of TNT. In other words, a bomb with a yield of 500 pounds will blow up with the same intensity as 500 pounds of TNT.

The "500-pound bombs" that fell on al-Zarqawi each had about 200 pounds of explosives. But their explosive yield would be 200 pounds only if they were filled with TNT. In fact, the Mk 82 makes use of a more advanced, TNT-based compound that produces a bit more energy when it blows up. Two hundred pounds of the fancy stuff might yield the equivalent of, say, 240 pounds of TNT.

Explosive yields come in handy when you're talking about nuclear weapons. A nuke that weighs about 1,200 pounds—or twice as much as the GBU-12 that was used the other day—can have an explosive yield of 14 kilotons. That's the equivalent of 14,000 metric tons of TNT, or almost 31 million pounds.